Microlensing and the structure of active galactic nucleus accretion disks

Rapid variability has been reported in two of the four gravitationally lensed images of Q2237+0305, and this is attributed to microlensing caused by the intervening stars. The associated constraints on the source size and properties are studied and compared with a variety of stationary accretion disk models. It is found that graybody disks have a half-power radius r~ 2 x 10¹6^ (V/VL_) ^-1. 7^ (LL, 46_) ⁰. 5^ (ɛ/0. 1) ^-0. 5 ᶜm, where V is the frequency, LL, 46_ is the luminosity per log frequency in units of 10⁴6^ ergs s^-1^ at the Lyman limit VL_, and ɛ is the emissivity relative to a blackbody. The reported microlensing variation in Q2237 + 0305 requires the disk size to be over 3 times smaller than a blackbody disk of similar luminosity, implying that the optical emission is either nonthermal or optically thin. An exploration of nonstationary disk models including orbiting, transient hot spots leads to a similar conclusion. Implications for models of active galactic nucleus optical continua are briefly discussed.